1. Field of the Invention
The present invention relates to a field effect transistor, and more particularly to a Schottky-gate field effect transistor capable of operating at a high speed and in a high frequency band.
2. Description of Related Art
At present, Schottky-gate field effect transistors, i.e., metal semiconductor field effect transistors (abbreviated as MESFETs hereinafter) have been widely used as microwave amplifiers and oscillators. Furthermore, the MESFETs have been used as fundamental elements for high speed digital integrated circuits.
Referring to FIG. 1, there is shown a conceptual construction of a basic MESFET. The shown MESFET includes a single-crystal substrate 1 of high-resistive or semi-insulative semiconductor material and an active layer 2 of conductive semiconductor crystal formed on the substrate 1. On the active layer 2 there are formed a drain electrode 3 and a source electrode 4 separately from each other an in ohmic contact to the active layer 2. In addition, a Schottky gate electrode 5 is deposited on the active layer 2 between the drain and source electrodes 3 and 4.
Such a MESFET has various parasitic capacitances between the electrodes, including the capacitance C.sub.gd formed between the drain 3 and the gate electrode 5, the capacitance C.sub.gs formed between the source 4 and the gate electrode 5, the capacitance C.sub.ds formed between the drain 3 and the source 4, etc. Among these parasitic capacitances, the gate-drain capacitance C.sub.gd has a substantial amount, compatible to the gate-source capacitance C.sub.gs, specifically about one-third of the gate-source capacitance C.sub.gs in the case of GaAs-MESFETs. Because of this, the gate-drain capacitance C.sub.ds causes a significant problem explained hereinafter.
Turning to FIG. 2, there is shown an exemplary circuitry of a source follower amplifier using the MESFET as mentioned above. The shown amplifier comprises an MESFET 6 having a gate connected to an input and a source connected to an output and grounded through a resistor R. A drain of the MESFET 6 is connected to a positive voltage source V.sub.cc. Also referring to FIG. 3, there is shown an example of a level shift circuit, which includes an MESFET 6 whose drain is connected to a positive voltage V.sub.CC and whose gate is connected to an input. A source of the MESFET 6 is connected through two diodes D1 and D2 to an output and a drain of another MESFET 7. A source of the MESFET 7 is connected to a biasing voltage V.sub.EE, and also connected to a gate thereof so that the MESFET 7 constitutes an active load functioning as a constant current source. The two circuits shown in FIGS. 2 and 3 are not only used as discrete circuits but also widely used as unitary circuits assembled in integrated circuits.
In the aforementioned circuits, assuming that the current gain is g.sub.m and the input capacitance is C.sub.in, the cutoff frequency f.sub.c is expressed as follows: EQU f.sub.c =(g.sub.m /C.sub.in).sup.n where n=1-2 (1)
In addition, the input capacitance C.sub.in is substantially equal to the gate-drain capacitance C.sub.dg. Therefore, the capacitance C.sub.dg is one significant factor which determines the performance of these circuits.
Generally, the input capacitance C.sub.in of amplifying circuits includes the gate-source capacitance C.sub.gs and the gate-drain capacitance C.sub.gd which functions as a feedback capacitance. looking at this gate-drain capacitance C.sub.gd from the input of the circuit, the capacitance C.sub.gd is substantially multiplied by the voltage gain G because of the so-called Miller effect. Namely, the input capacitance C.sub.in is expressed as follows: EQU C.sub.in =C.sub.gs +(1-G) C.sub.gd ( 2)
Now recalling that the gate-drain capacitance C.sub.gd is about one-third of the gate-source capacitance C.sub.gs, and considering that an ordinary GaAs-MESFET amplifier has a voltage gain G of at least 10 times, the input capacitance C.sub.in is substantially dominated by the feedback capacitance (1-G)C.sub.gd, i.e., the gate-drain capacitance C.sub.gd. As a result, the cutoff frequency f.sub.c is determined by the gate-drain capacitance C.sub.gd, and therefore, has been limited to 1.2 GHz in the conventional GaAs-MESFET amplifier.
As seen from the above, in the circuits shown in FIGS. 2 and 3, the gain at high frequencies, the cutoff frequency, the input impedance, and the operation speed (in the case of logical circuit) are determined by the current gain and the gate-drain capacitance C.sub.gd of the MESFET used. Therefore, decrease in the gate-drain capacitance C.sub.gd of MESFETs is very important to improvement in the characteristics of the circuits.
For the purpose of reducng the gate-drain capacitance C.sub.gd of MESFETs, it is considered to use a dual-gate MESFET provided with an elaborate external circuit, as a single-gate MESFET having an equivalently small gate-drain capacitance C.sub.gd.
Referring to FIG. 4, there is shown a conceptual structure of an exemplary dual-gate MESFET. The shown dual-gate MESFET comprises a substrate 1 and an active layer 2 formed thereon. A drain electrode 3 and a source electrode 4 are deposited on the active layer 2, and also, a pair of Schottky gate electrodes 8 and 9 are deposited on the active layer portion between the drain and source electrodes 3 and 4.
If the dual-gate MESFET is used as an amplifier, the Schottky gate electrode 8 adjacent to the drain electrode 3 is short-circuited to the source electrode 4 through an external circuit (not shown) and a signal is applied to only the Schottky gate electrode 9 adjacent to the source electrode 4.
With such connection, the capacitance between the Schottky gate electrode 9 and the drain electrode 3 is decreased to a few tenths or less of that of the conventional single-gate MESFET.
However, a new substantial problem will arise from parasitic inductances and capacitances attributable to the external circuit provided to the dual-gate MESFET. In addition, the conventional dual-gate MESFET has a current gain smaller than a single-gate MESFET and cannot be properly biased only with a simple connection between the source and the second gate adjacent to the drain, so that the aforementioned circuit of the dual-gate MESFET is difficult to provide good high speed operability and good gain at high frequencies.